It is generally known to supply commercial aircraft with conditioned air for heating and cooling when the aircraft is stationary at a gate. In this application, the term gate is meant to refer to any place that an aircraft receives or discharges passengers or cargo. This may be by way of a telescoping corridor (also referred to as a walkway, bridge way, jet bridge), stairs, or any other facility. Typically, conditioned air is supplied to the aircraft from a pre-conditioned air (PCA) unit associated with the gate that is a part of the airport terminal. The PCA unit may produce heated air or cooled air depending on the needs of the aircraft it is servicing. The PCA outputs its air into a duct that may be rigid or flexible, and then the air is delivered from the gate to the aircraft with a flexible and usually insulated air hose. When not in use, the hose is usually stored under the terminal. In some installations a branch of the same duct, or a separate one, may be used to supply preconditioned air to the enclosed walkway that passengers walk through to access the airplane. In such a case, there may also be a return air duct from the walkway to the PCA unit, or there may not be one.
A problem arises if the PCA unit starts to malfunction and fails to supply the cooled air or the heated air of which it is capable. It is easy for this condition to go undetected, because a PCA unit is not used like a typical building heating, ventilation, and cooling (HVAC) unit. In the latter, the HVAC is permanently connected and typically running per the commands of a thermostat that is sensing room air temperature. The HVAC unit services a building of a given size, with a permanent ducting system that is never kinked, improperly connected, or torn. Usually there is a steady group of occupants, and a designated maintenance person to pay attention to the HVAC system. If the occupants become uncomfortable, it is quickly evident that the HVAC system is probably not cooling or heating to its usual ability, and corrective action is taken. But the situation with a PCA unit is different. For example, the PCA unit is called upon to service aircraft of different sizes. The flexible hose is often kinked because usually a single length hose is used to hook up aircraft with connections at varying distances from the PCA unit. The hose may be torn. There may be a delay in how quickly the PCA unit is hooked up to the aircraft and turned on. It may only be hooked up for a short time. Somebody may fail to turn the PCA unit on. The PCA unit may be switched off overnight even though an aircraft is parked at the gate, and the aircraft heats up in the sun the next morning, or cools down excessively overnight. For these and various other reasons a complaint of “too hot” or “too cold” by the people using the aircraft and the walkway may be considered of limited value by the ground based personnel who have to keep many PCA units operating. A decrease in operating performance by a PCA unit is likely to go unnoticed and unattended to by the people who could fix it before complete failure.